This information can be found at:
http://www.sos.state.il.us:80/depts/archives/data_lan.htmlThe link is for the main page which also gives information on how
you can get copies of the records themselves. Also you can click
the first link from that page and it will take you to a searchable database
so you can search for your other ancestors in IL. Below in the records
under county is a number. If you go to the searchable database page
and click bottom link for "Table of County Codes" you can look them up
by number and it will give the county. Good luck!